JP7708472B2 - Applied magnetic field synthesis and processing of iron nitride magnetic materials - Google Patents

Description

RELATED APPLICATIONS This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/107,700, entitled "APPLIED MAGNETIC FIELD SYNTHESIS AND PROCESSING OF IRON NITRIDE MAGNETIC MATERIALS," filed on January 26, 2015, the entire contents of which are incorporated herein by reference.

This disclosure relates to techniques for forming iron nitride magnetic materials.

Permanent magnets play a role in many electromechanical systems, including, for example, alternative energy systems. For example, permanent magnets are used in sensors, actuators, electric motors, or generators, which can be used in transportation, wind turbines, and other alternative energy mechanisms. Many permanent magnets currently in use contain rare earth elements, such as neodymium, which provide high energy products. These rare earth elements are in relatively short supply and may face rising prices and/or supply shortages in the future. Additionally, certain permanent magnets containing rare earth elements are expensive to manufacture. For example, the manufacture of NdFeB and ferrite magnets typically involves grinding materials, compressing materials, and sintering at temperatures in excess of 1000° C., all of which contribute to the high manufacturing costs of magnets. Additionally, the mining of rare earth elements can result in significant environmental degradation.

The present disclosure describes techniques for forming a magnetic material including at least one iron-based phase domain including uniaxial magnetic anisotropy. For example, the iron-based phase domain including uniaxial magnetic anisotropy may include iron having a body-centered tetragonal structure, α″-Fe ₁₆ N ₂ , α″-Fe ₁₆ C ₂ , Fe, or other Fe-based magnetic materials. The techniques described herein may include at least one of casting a mixture of iron and nitrogen in an applied magnetic field or consolidating a plurality of workpieces, at least some of which include at least one iron-based phase domain including uniaxial magnetic anisotropy, while exposing the plurality of workpieces to an applied magnetic field.

During the casting technique, iron nitride crystals may nucleate and grow from a molten mixture containing iron and nitrogen. By applying a magnetic field during the casting process, the nucleation and growth of the iron nitride crystals may be influenced such that the growth of crystals having a given orientation may be energetically favored. For example, iron nitride crystals having (002) or (004) crystal faces substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field may be more energetically favored than iron nitride crystals having a different orientation (e.g., (110), (112), (202) or (200) crystal faces substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field). Thus, the applied magnetic field may increase the likelihood that some or all of the iron nitride crystals have a similar crystal orientation. A material including a plurality of iron nitride crystals having a substantially similar crystal orientation may increase the magnetic anisotropy of the material.

During synthesis, e.g. α″-Fe ₁₆ N A magnetic field may be applied to the material being consolidated to substantially align (e.g., align or nearly align (e.g., within about 5 degrees of perfect alignment)) the magnetic easy axes of the plurality of workpieces including at least _one iron-based phase domain including uniaxial magnetic anisotropy such as .2. The magnetic easy axis is the direction of the iron-based phase domain crystal cells in which alignment of the magnetic moments is energetically favorable and metastable. In some examples, the magnetic easy axis of the iron-based phase domain including the uniaxial magnetic anisotropy unit cell is the <001> or c-axis. In some examples, the plurality of workpieces may include powders, particles, ribbons, sheets, wires, or other geometric shapes. By applying a magnetic field during the compaction process, the magnetic easy axes of the plurality of workpieces including at least one iron-based phase domain including uniaxial magnetic anisotropy may be aligned substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field. This helps define the magnetization direction of the consolidated magnetic material and can increase the magnetic anisotropy of the consolidated magnetic material.

In some examples, the present disclosure describes a method that includes casting an iron-containing material in the presence of an applied magnetic field to form a workpiece that includes at least one iron-based phase domain that includes uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T).

In some examples, the present disclosure describes a method that includes compressing a plurality of workpieces, each workpiece including at least one iron-based phase domain including uniaxial magnetic anisotropy, in the presence of an applied magnetic field to form a bulk material including a plurality of iron-based phase domains including uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T), the applied magnetic field defining a magnetization direction of the bulk material.

In some examples, the present disclosure describes an apparatus configured to implement any of the techniques described herein.

In some examples, the present disclosure describes a workpiece formed by any of the techniques described herein.

In some examples, the present disclosure describes bulk materials formed by any of the methods described herein.

In some examples, the present disclosure describes a method that includes casting a material that includes at least one of nickel, iron, and cobalt in the presence of an applied magnetic field to form a workpiece that includes at least one nickel-, iron-, or cobalt-based phase domain that includes uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T).

In some examples, the disclosure describes a workpiece including at least one anisotropically shaped iron-based grain, the at least one anisotropically shaped iron-based grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension to the length of the shortest dimension of the anisotropic grain. The longest and shortest dimensions may be substantially orthogonal.

In some examples, the present disclosure describes bulk permanent magnets that include at least one anisotropically shaped iron-based grain, the at least one anisotropically shaped iron-based grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension of the anisotropic grain to the length of the shortest dimension. The longest and shortest dimensions may be substantially orthogonal.

The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will become apparent from the description and drawings, and from the claims.

FIG. 1 is a flow diagram illustrating an exemplary technique for casting iron and nitrogen containing materials in the presence of an applied magnetic field. FIG. 2 is a schematic diagram illustrating an exemplary system for performing a casting technique on a mixture containing iron and nitrogen using an RF furnace, a crucible, and an optional quenching medium. FIG. 3 is a conceptual diagram illustrating an exemplary system including a crucible heating stage that can be used to cast iron and nitrogen containing materials in the presence of an external magnetic field. FIG. 4 is a schematic diagram showing further details of one example of the crucible heating step shown in FIG. FIG. 5 is a conceptual diagram illustrating another example system for belt casting an exemplary iron nitride workpiece in the presence of an external magnetic field. FIG. 6 is a conceptual diagram showing an α″-Fe ₁₆ N ₂ unit cell. FIG. 7 is a conceptual diagram illustrating an exemplary α″-Fe ₁₆ N ₂ crystal or grain having an anisotropic shape. FIG. 8 is a conceptual diagram illustrating an exemplary workpiece including a plurality of α″-Fe ₁₆ N ₂ crystals or grains in a matrix of another material. FIG. 9 illustrates an example hysteresis curve for the example workpiece shown in FIG. FIG. 10 is a flow diagram illustrating an exemplary technique for consolidating multiple workpieces including at least one α″-Fe ₁₆ N _two -phase domain to form a bulk magnetic material. FIG. 11 is a flow diagram illustrating an exemplary technique for forming a bulk magnetic material containing α″-Fe ₁₆ N _two -phase domains from raw materials containing iron and nitrogen. FIG. 12 shows exemplary x-ray diffraction spectra from iron nitride material cast with and without the application of an external magnetic field.

The present disclosure may be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawings and examples, which form a part of this disclosure. The present disclosure is not limited to the specific devices, methods, applications, conditions, or parameters described and/or illustrated herein, and the terms used herein are for the purpose of describing specific examples and are not intended to limit the scope of the claims. When a range of values is expressed, other examples include the one specific value and/or the other specific value. Similarly, when values are expressed as approximations, by use of the prefix "about," it will be understood that the specific value constitutes another example. All ranges are inclusive and combinable. Furthermore, references to values recited within a range include each value within that range.

It should be understood that certain features of the disclosure that are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, for brevity, various features of the disclosure that are described in the context of a single embodiment may also be provided separately or in any subcombination.

The present disclosure relates to magnetic materials including at least one iron-based phase domain with uniaxial magnetic anisotropy, bulk permanent magnets including at least one iron-based phase domain with uniaxial magnetic anisotropy, techniques for forming magnetic materials including at least one iron-based phase domain with uniaxial magnetic anisotropy, and techniques for forming bulk permanent magnets including at least one iron-based phase domain with uniaxial magnetic anisotropy. Bulk permanent magnets including at least one iron-based phase domain with uniaxial magnetic anisotropy can provide an alternative to permanent magnets including rare earth elements, since iron-based phase domains including uniaxial magnetic anisotropy can have high saturation magnetization, high magnetic anisotropy constant, and therefore high energy product. An example of an iron-based compound including uniaxial magnetic anisotropy is α″-Fe ₁₆ N _2. Other examples of iron-based compounds include those having a body-centered tetragonal crystal structure, such as distorted iron, and some compounds including iron and at least one of N, C, B, O, P, Y, Mn, Co, Cr, Si, Al, Zn, etc.

α″-Fe ₁₆ N ₂ has a high saturation magnetization, a high magnetic anisotropy constant, and therefore a high energy product. The high saturation magnetization and magnetic anisotropy constant result in a magnetic energy product that can be higher than rare earth magnets in some instances. Bulk α″-Fe ₁₆ N ₂ permanent magnets produced in accordance with the techniques described herein can have desirable magnetic properties, such as a high energy product of about 130 MGOe when the α″-Fe ₁₆ N ₂ permanent magnet is anisotropic. In examples where the α″-Fe ₁₆ N ₂ magnet is isotropic, the energy product can be as high as about 33.5 MGOe. The energy product of a permanent magnet is proportional to the product of the remanent coercivity and the remanent magnetization. The energy product of this permanent magnet is proportional to the product of the remanent magnetization and the remanent magnetization. By way of comparison, the energy product of a Nd ₂ Fe ₁₄ B permanent magnet can be as high as about 60 MGOe. A higher energy product can result in improved efficiency of the permanent magnet when used in sensors, actuators, motors, generators, etc. Furthermore, permanent magnets including the Fe ₁₆ N ₂ phase may be free of rare earth elements, which can reduce the material cost of the magnet and reduce the environmental impact of manufacturing the magnet.

Without being limited to any theory of operation, α″-Fe ₁₆ N ₂ is believed to be a metastable phase that competes with other stable phases of iron nitride. Thus, it can be difficult to form bulk magnetic materials and bulk permanent magnets that include an α″-Fe ₁₆ N ₂ phase domain. Various techniques described herein can facilitate the formation of a magnetic material that includes an Fe ₁₆ N ₂ iron nitride phase domain. In some instances, the techniques can reduce the cost of forming a magnetic material that includes an α″-Fe ₁₆ N ₂ iron nitride phase domain, increase the volume fraction of the α″-Fe ₁₆ N ₂ iron nitride phase domain in the magnetic material, facilitate mass production of a magnetic material that includes an Fe ₁₆ N ₂ iron nitride phase domain, and/or improve the magnetic properties of a magnetic material that includes an Fe ₁₆ N ₂ iron nitride phase domain, as compared to other techniques for forming a magnetic material that includes an Fe ₁₆ N ₂ iron nitride phase domain.

The bulk permanent magnets described herein that include at least one iron-based phase domain that includes uniaxial magnetic anisotropy, such as, for example, α″-Fe ₁₆ N _2, can have anisotropic magnetic properties that are characterized as having different energy products, coercivities, and magnetization moments at different relative orientations with respect to an applied electric or magnetic field. Accordingly, the disclosed bulk iron nitride magnets can be used in any of a variety of applications, such as electric motors, to provide low energy losses and high energy efficiency for such applications.

The present disclosure describes techniques for forming a magnetic material including at least one iron-based phase domain including uniaxial magnetic anisotropy, such as at least one α″-Fe ₁₆ N ₂ -phase domain. The techniques described herein can include at least one of casting a mixture of iron and nitrogen in an applied magnetic field or consolidating a plurality of workpieces including at least one iron-based phase domain, at least some of which include uniaxial magnetic anisotropy, such as at least one α″-Fe ₁₆ N ₂ -phase domain, while exposing the plurality of workpieces to an applied magnetic field.

During the casting technique, iron nitride crystals may nucleate and grow from a molten mixture containing iron and nitrogen. By applying a magnetic field during the casting process, the nucleation and growth of the iron nitride crystals may be influenced such that the growth of crystals having a given orientation may be energetically favored. For example, iron nitride crystals having (002) or (004) crystal faces substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field may be energetically favored over iron nitride crystals having a different orientation (e.g., (110), (112), (202) or (200) crystal faces substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field). Thus, the applied magnetic field may increase the likelihood that some or all of the iron nitride crystals have a similar crystal orientation. A material including a plurality of iron nitride crystals having a substantially similar crystal orientation may increase the magnetic anisotropy of the material.

In some instances, in addition to having uniaxial magnetic anisotropy, the casting technique can form at least one iron nitride crystal or grain that exhibits an anisotropic shape. The at least one anisotropically shaped iron nitride crystal or grain can exhibit an aspect ratio of about 1.1 to about 50, such as about 1.4 to about 50, or 2.2 to about 50, or about 5 to about 50, or such as about 5 to about 50. As used herein, aspect ratio is defined as the ratio of the length of the longest dimension to the length of the shortest dimension of the anisotropic grain, where the shortest dimension is measured in a direction that is substantially perpendicular (e.g., perpendicular or nearly perpendicular (e.g., within about 5 degrees of perpendicular)) to the longest dimension. In some instances, the longest dimension of the at least one anisotropically shaped iron nitride crystal or grain is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field, and thus to the direction of the uniaxial magnetic anisotropy. Similarly, the longest dimension of at least one anisotropically shaped iron nitride crystal or grain can be parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the easy axis of the magnetocrystalline anisotropy of the anisotropically shaped iron nitride crystal or grain. For example, for body-centered tetragonal (bct) Fe ₁₆ N ₂ and Fe, the (002) texture is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the longest dimension of the crystal or grain. In this manner, the shape anisotropy possessed by the anisotropically shaped iron nitride crystal or grain can contribute to the magnetic anisotropy of the material. In another example, for (bc)Fe ₁₆ N ₂ , the (002) texture can be substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the shortest dimension of the crystal or grain.

During consolidation, a magnetic field may be applied to the material being consolidated _to substantially (e.g., aligned or nearly aligned (e.g., within about ₅ degrees of perfect alignment)) the magnetic easy axes of the plurality of workpieces including at least one iron-based phase domain including uniaxial magnetic anisotropy, such as α″-Fe 16 N 2 . The magnetic easy axis is the direction of a crystal cell in which alignment of magnetic moments is energetically favorable and metastable. In some examples, the magnetic easy axis of a unit cell of an iron-based phase domain including uniaxial magnetic anisotropy is the <001> or c-axis. In some examples, the plurality of workpieces may include powders, particles, ribbons, sheets, wires, or other geometric shapes. By applying a magnetic field during the compaction process, the magnetic easy axes of the plurality of workpieces including at least one iron-based phase domain including uniaxial magnetic anisotropy may be aligned substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field. This helps define the magnetization direction of the consolidated magnetic material and can increase the magnetic anisotropy of the consolidated magnetic material.

In some examples, the casting and consolidation techniques can be used together as part of a larger scale technique to form a bulk magnetic material comprising at least one iron-based phase domain comprising a uniaxial magnetic anisotropy α″-Fe ₁₆ N ₂ -phase domain. In some examples, this larger scale technique can include additional steps including, for example, quenching the cast magnetic material, annealing the quenched magnetic material, etc. In some examples, an external magnetic field can be applied during at least a portion of these other steps to promote the formation of at least one iron-based phase domain comprising a uniaxial magnetic anisotropy α″-Fe ₁₆ N ₂ -phase domain. For example, a magnetic field may be applied during the annealing step to facilitate the formation of at least one iron-based phase domain, including a uniaxial magnetic anisotropy α″-Fe ₁₆ N ₂ phase domain, in the material. The following description primarily describes iron nitride materials such as α″-Fe ₁₆ N ₂ , but one of ordinary skill in the art will appreciate that the following description also applies to other iron-based materials that include uniaxial magnetic anisotropy, such as distorted iron or iron-based materials that include at least one of N, C, B, O, P, Y, Mn, Co, Cr, Si, Al, Zn, etc., and have a body-centered tetragonal structure.

FIG. 1 is a flow diagram illustrating an exemplary technique for casting a material comprising iron and nitrogen in the presence of an applied magnetic field. The technique of FIG. 1 includes forming a molten mixture comprising iron and nitrogen (12). The molten mixture can be formed using any one of a number of techniques. For example, a solid material comprising iron and nitrogen can be first formed, and then the solid material comprising iron and nitrogen can be melted to form the molten mixture comprising iron and nitrogen. As another example, molten iron can be mixed with a nitrogen source to form the molten mixture comprising iron and nitrogen.

An exemplary technique for forming a solid material containing iron and nitrogen includes nitriding an iron-containing workpiece. The iron-containing workpiece may be, for example, a powder, particle, ribbon, sheet, wire, or other geometric shape. In some examples, nitriding the iron-containing workpiece may include heating the iron-containing workpiece for a time sufficient to diffuse nitrogen to a predetermined concentration substantially throughout the iron-containing workpiece. Thus, the heating time and temperature are related and may also be influenced by the composition and/or geometry of the iron-containing workpiece. For example, iron wire or sheet 28 may be heated to a temperature of about 125° C. to about 600° C. for about 2 hours to about 9 hours.

In addition to heating the iron-containing workpiece, nitriding the iron-containing workpiece includes exposing the iron-containing workpiece to an atomic nitrogen substance that diffuses into the iron-containing workpiece. In some examples, the atomic nitrogen substance is provided as diatomic nitrogen ( _N2 ), which is then dissociated (decomposed) into individual nitrogen atoms. In other examples, the atomic nitrogen may be provided from other atomic nitrogen precursors, such as ammonia ( _NH3 ). In other examples, the atomic nitrogen may be provided from urea (CO( _NH2 ) ₂ ). The nitrogen may be provided in the gas phase alone (e.g., substantially pure ammonia or diatomic nitrogen gas) or as a mixture with a carrier gas. In some examples, the carrier gas is argon (Ar).

In some examples, nitriding the iron-containing workpiece may include a urea diffusion process in which urea is utilized as the nitrogen source (e.g., rather than diatomic nitrogen or ammonia). Urea (also called carbamide) is an organic compound having the chemical formula CO( _NH2 ) ₂ . To nitride the iron-containing workpiece, urea can be heated, for example, in a furnace surrounding the iron-containing workpiece to generate decomposed nitrogen atoms that can diffuse into the iron-containing workpiece. As described further below, the composition of the resulting iron nitride material can be controlled to some extent by the temperature of the diffusion process and the ratio (e.g., mass ratio) of iron-containing workpiece to urea used for the process. Further details regarding these nitriding processes (including urea diffusion) can be found in International Patent Application No. PCT/US12/51382, filed August 17, 2012, the entire contents of which are incorporated herein by reference.

As another example of forming a solid material containing iron and nitrogen, nitrogen may be generated using a plasma, such as an RF plasma or a DC plasma, from a nitrogen source, such as a gaseous nitrogen source. The iron-containing workpiece is placed in a plasma environment, such as a plasma chamber, and nitrogen atoms generated by the plasma process are implanted into and allowed to diffuse into the iron-containing workpiece.

As another example of forming a solid material containing iron and nitrogen, ion implantation can be used to implant nitrogen atoms into an iron-containing workpiece. For example, the iron-containing workpiece can be a foil. The foil can exhibit a thickness on the order of hundreds of nanometers to several millimeters. In some examples, the foil can exhibit a thickness of about 500 nanometers (nm) to about 1 millimeter (mm). The thickness of the foil can affect the parameters used to ion implant and anneal the foil, as described below. The thickness of the foil can be measured in a direction substantially perpendicular (e.g., perpendicular or nearly perpendicular (e.g., within about 5 degrees of perpendicular)) to the surface of the substrate to which the foil is attached.

The average depth to which N+ ions are implanted into an iron-containing workpiece may depend on the energy to which the N+ ions are accelerated. In general, the average implantation depth of the N+ ions may increase with increasing implantation energy.

The implantation energy used to implant the N+ ions can be selected based at least in part on the thickness of the iron-containing workpiece. The implantation energy can also be selected to implant the N+ ions without causing excessive damage to the iron-containing workpiece, including the crystal lattice of the iron crystals in the iron-containing workpiece. For example, a higher implantation energy can allow for implantation of the N+ ions at a greater average depth, but the higher implantation energy can increase damage to the iron workpiece, including damaging the crystal lattice of the iron crystals and ablating some of the iron atoms due to the bombardment of the N+ ions. Thus, in some examples, the implantation energy can be limited to less than about 180 keV. In some examples, the incidence angle of the implantation can be about 0 degrees (e.g., substantially normal (e.g., parallel or near normal (e.g., within about 5 degrees of normal)) to the surface of the iron workpiece). In other examples, the incidence angle of the implantation can be adjusted to reduce lattice damage. For example, the incidence angle of the implantation can be about 3° to about 7° from normal.

As an example, if the iron-containing workpiece exhibits a thickness of about 500 nm, an implant energy of about 100 keV can be used to implant N+ ions into the iron-containing workpiece. An implant energy of about 100 keV can also be used to implant N+ ions into iron-containing workpieces of other thicknesses. In other examples, different implant energies can be used for iron-containing workpieces exhibiting a thickness of about 500 nm, and the same or different implant energies can be used for iron-containing workpieces exhibiting thicknesses different from 500 nm.

Further, the flow rate of the N+ ions can be selected to implant a desired amount of N+ ions into the iron-containing workpiece. In some examples, the flow rate of the N+ ions can be selected to implant an approximately stoichiometric number of N+ ions into the iron-containing workpiece. The stoichiometric ratio of iron to nitrogen in Fe ₁₆ N ₂ is 8:1. Thus, the approximate number of iron atoms in the iron-containing workpiece can be determined, and a number of N+ ions equal to about ⅛ (12.5%) of the iron atoms, such as about 8 atomic % to about 15 atomic %, can be implanted into the iron-containing workpiece. For example, an iron-containing workpiece having dimensions of about 1 cm×1 cm×500 nm can contain about 4.23×10 ¹⁸ iron atoms. Thus, to achieve a stoichiometric ratio of iron atoms to N+ ions in the iron-containing workpiece, about 5.28×10 ¹⁷ N+ ions can be implanted into the sample.

The temperature of the iron-containing workpiece during ion implantation can also be controlled. In some examples, the temperature of the iron-containing workpiece can be from about room temperature to about 500° C. Further details regarding the implantation of N+ ions into the iron-containing workpiece can be found in International Application No. PCT/US14/15104, filed February 6, 2014, the entire contents of which are incorporated herein by reference.

Another example technique for forming a solid material containing iron and nitrogen includes grinding an iron-containing material, such as a powder, in the presence of a nitrogen source. Grinding equipment used to grind the iron-containing material includes rolling, stirring, or vibration mode grinding equipment. Grinding equipment includes a bin that encloses the iron-containing material, the nitrogen source, and grinding media.

Grinding media can include, for example, grinding balls. The grinding media can include a material that is hard enough to wear down the iron-containing material when it contacts the iron-containing material with sufficient force, reducing the particles of the iron-containing material to a smaller average size. In some examples, the grinding media can be formed of steel, stainless steel, and the like. In some examples, the material from which the grinding media is formed does not chemically react with the iron-containing material and/or the nitrogen source.

The iron-containing material may include any material containing, for example, atomic iron, iron oxide, iron chloride, etc. In some examples, the iron-containing material may include substantially pure iron (e.g., iron containing less than about 10 atomic percent (at.%) of a dopant or impurity). In some examples, the dopant or impurity may include oxygen or iron oxide.

The nitrogen source may include ammonium nitrate (NH ₄ NO ₃ ) or an amide-containing material, such as a liquid amide or a solution containing an amide, or hydrazine or a solution containing hydrazine. Amides contain a C-N-H bond and hydrazine contains an N-N bond. Ammonium nitrate, amides and hydrazines act as nitrogen donors to form powders containing iron nitride. Example amides include carbamide ((NH ₂ ) ₂ CO; also known as urea), methanamide, benzamide, and acetamide, although any amide may be used. In some instances, amides may be derived from carboxylic acids by replacing the hydroxyl group of the carboxylic acid with an amine group. This type of amide may be referred to as an acid amide.

In some instances, the grinding machine bin can also contain a catalyst. The catalyst can include, for example, cobalt (Co) particles and/or nickel (I) particles. The catalyst catalyzes the nitridation of iron-containing materials. One possible conceptualized reaction pathway for nitriding iron using a Co catalyst is shown below in reactions 1-3. A similar reaction pathway can be followed when using Ni as the catalyst.

Thus, by mixing sufficient amide and catalyst 22, the iron-containing feedstock 18 can be converted to an iron nitride-containing material. Further details regarding grinding an iron-containing material in the presence of a nitrogen source to form a solid material containing iron and nitrogen can be found in International Application No. PCT/US14/43902, filed June 24, 2014, which is incorporated herein by reference.

Regardless of the technique by which the iron and nitrogen containing solid material is formed, the iron and nitrogen containing solid material can include an iron to nitrogen atomic ratio of about 8:1. For example, the mixture can include about 8 atomic percent (at.%) to about 15 at.% nitrogen, with the remainder being iron, other elements, and dopants. As another example, the mixture can include about 10 at.% to about 13 at.% nitrogen, or about 11.1 at.% nitrogen.

In some examples, the iron and nitrogen containing mixture may include, in addition to iron and/or nitrogen, at least one iron nitride, such as FeN, Fe ₂ N (e.g., ξ-Fe ₂ N), Fe ₃ N (e.g., ε-Fe ₃ N), Fe ₄ N (e.g., γ'-Fe ₄ N and/or γ-Fe ₄ N), Fe ₂ N ₆ , Fe ₈ N, Fe ₁₆ N ₂ , or FeN _x (where x is from about 0.05 to about 0.5). In some examples, the iron and nitrogen containing mixture may have a purity (e.g., collective iron and nitrogen content) of at least 92 atomic percent (at.%).

In some examples, the mixture containing iron and nitrogen can include at least one dopant, such as a ferromagnetic or non-magnetic dopant and/or a phase stabilizer. In some examples, the at least one ferromagnetic or non-magnetic dopant can be referred to as a ferromagnetic or non-magnetic impurity and/or the phase stabilizer can be referred to as a phase stabilizing impurity. The ferromagnetic or non-magnetic dopant can be used to increase at least one of the magnetic moment, coercivity, or thermal stability of the magnetic material formed from the mixture containing iron and nitrogen. Examples of ferromagnetic or non-magnetic dopants are selected from the group consisting of Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Ca, Pt, Au, Sm, C, Pb, W, Ga, Y, Mg, Hf, and Ta. For example, the inclusion of Mn dopant atoms at a level of about 5 at.% to about 15 at.% in an iron nitride material including at least one Fe ₁₆ N ₂ -phase domain can improve the thermal stability of the Fe ₁₆ N ₂ -phase domain and the coercivity of the material as compared to an iron nitride material that does not include Mn dopant atoms. In some examples, two or more (e.g., at least two) ferromagnetic or non-magnetic dopants including iron and nitrogen can be included in the mixture. In some examples, the ferromagnetic or non-magnetic dopants can function as domain wall pinning sites and improve the coercivity of a magnetic material formed from a mixture including iron and nitrogen. Table 1 includes examples of ferromagnetic or non-magnetic dopant concentrations in a mixture including iron and nitrogen.

Alternatively or additionally, the mixture containing iron and nitrogen may include at least one phase stabilizer. The at least one phase stabilizer may be an element selected to improve at least one of the Fe ₁₆ N ₂ volume ratio, thermal stability, coercivity, and erosion resistance. When present in the mixture, the at least one phase stabilizer may be present in the mixture containing iron and nitrogen at a concentration of about 0.1 at.% to about 15 at.%. In some examples where at least two phase stabilizers are present in the mixture, the total concentration of the at least two phase stabilizers may be about 0.1 at.% to about 15 at.%. The at least one phase stabilizer may include, for example, B, Al, C, Si, P, O, Co, Cr, Mn, and/or S. For example, the inclusion of Mn dopant atoms at a level of about 5 at.% to about 15 at.% in an iron nitride material containing at least one Fe ₁₆ N ₂ -phase domain can improve the thermal stability of the Fe ₁₆ N ₂ -phase domain and the magnetic coercivity of the material as compared to an iron nitride material that does not contain the Mn dopant atoms.

Alternatively, instead of forming a solid material containing iron and nitrogen, a nitrogen source may be mixed with the molten iron to form a molten mixture containing iron and nitrogen. Further details regarding mixing the nitrogen source with the molten iron are illustrated and described below with respect to FIG. 5.

In some examples, instead of forming a molten material (12) that includes iron and nitrogen, the technique of FIG. 1 may include forming a molten material that includes only iron, or forming a molten material that includes iron and at least one of N, C, B, O, P, Y, Mn, Co, Cr, Si, Al, Zn, etc., in relative proportions such that at least some of the material forms a body-centered tetragonal structure upon casting.

The technique of FIG. 1 also includes casting a material containing iron and nitrogen in the presence of an applied magnetic field (14). Also, FIGS. 2-4 show an exemplary apparatus that can be used to cast a material containing iron and nitrogen in the presence of an applied magnetic field.

2 is a conceptual diagram illustrating an exemplary system 20 for performing a casting technique on an iron-nitrogen containing mixture utilizing an RF furnace 22, a crucible 26, and an optional quenching medium 28. The system 20 includes an RF furnace 22 that surrounds a crucible 26. The crucible may be formed of a material that is thermally stable at temperatures within the RF furnace 22 during heating of the iron-nitrogen containing mixture. For example, the crucible 26 may include one or more refractory materials, such as graphite, refractory ceramics, etc.

The RF furnace 22 also includes an RF source 24, shown in FIG. 2 as a number of coils used to generate an RF magnetic field and heat at least the iron-nitrogen containing mixture in the crucible 26. In some examples, the RF source 24 can generate RF energy having a frequency of about 13.56 GHz or about 900 MHz in some examples. The RF source 24 can inductively heat the iron-nitrogen containing mixture directly or by heating a structure (e.g., the crucible 26) in the RF furnace 22 and then heating the iron-nitrogen containing mixture. The iron-nitrogen containing mixture can be heated in the RF furnace 22 above the melting temperature of the iron-nitrogen containing mixture to form a molten iron-nitrogen containing mixture.

In some examples, the shape of the crucible 26 can define the shape of the iron and nitrogen containing mixture, such as at least one wire, ribbon, or other article having a length greater than its width or diameter. In some examples, the temperature of the crucible 26 can be maintained at a temperature of about 650°C to about 1200°C during the casting process. In some examples, the temperature of the crucible 26 can be maintained at a temperature of about 800°C to about 1200°C during the casting process. The casting process can be carried out in air, a nitrogen environment, an inert environment, a partial vacuum, a full vacuum, or any combination thereof. The casting process can be at any pressure, such as about 0.1 GPa to about 20 GPa.

The system 20 also includes a magnetic field generator 30 that generates an external magnetic field 32 to which the RF furnace 22 and the material (e.g., the molten mixture comprising iron and nitrogen) in the RF furnace are exposed. The external magnetic field 32 can be applied to the material comprising iron and nitrogen while the molten mixture comprising iron and nitrogen cools to a solid material. In some examples, the external magnetic field 32 may be applied throughout the time the mixture comprising iron and nitrogen is melting. In some examples, the external magnetic field 32 may be applied while the solid material comprising iron and nitrogen is melted to form the molten mixture comprising iron and nitrogen.

The external magnetic field 32 may affect the nucleation and growth of crystal grains during cooling and solidification of the molten mixture of iron and nitrogen into a solid mixture of iron and nitrogen. For example, without wishing to be bound by any theory of action, the Gibbs free energy of a crystal grain may depend on its orientation with respect to the external magnetic field 32. For example, a crystal grain whose (002) or (004) plane is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the external magnetic field 32 may have a lower Gibbs free energy than a crystal grain whose (110), (112), (202) or (200) plane is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the external magnetic field 32. Thus, a crystal grain is more likely to nucleate and grow with its (002) or (004) plane substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the external magnetic field 32. This can promote substantial orientation (e.g., aligned or nearly aligned) of the crystallographic axes of the iron or iron nitride crystals formed during the casting process, which can promote having the crystallographic axes of the α″-Fe ₁₆ N ₂ substantially aligned (e.g., aligned or nearly aligned (e.g., within about 5 degrees of alignment)) when the α″-Fe ₁₆ N ₂ is formed.

Additionally or alternatively, the external magnetic field 32 may promote the diffusion of nitrogen into the interstitial spaces in the iron lattice, thereby reducing or substantially preventing the diffusion of nitrogen out of the iron and nitrogen containing material. While not wishing to be bound by any theory of operation, it is currently believed that the external magnetic field 32 may interact with the iron crystal lattice and distort the crystal lattice as the iron crystals nucleate and grow. The distortion of the iron crystal lattice will allow nitrogen to diffuse more easily into the interstitial spaces in the iron lattice. Once nitrogen diffuses into the interstitial spaces in the iron lattice, it will be more difficult for the nitrogen to diffuse out of the iron lattice. Additionally or alternatively, while not wishing to be bound by any theory of operation, it is currently believed that the external magnetic field 32 may weaken convection in the molten iron nitride mixture, thereby reducing the movement of nitrogen atoms forward of the solid-liquid interface during the growth of the iron nitride crystals.

Because the external magnetic field 32 can affect the nucleation density and defect density during the nucleation and growth process, the external magnetic field 32 can also affect the iron grain size, grain size uniformity, grain boundaries, and grain shape. For example, due to the application of the external magnetic field 32, in addition to having uniaxial magnetic anisotropy, a workpiece formed by casting a material comprising iron and nitrogen in the presence of an applied magnetic field (14) can include at least one iron nitride crystal or grain exhibiting an anisotropic shape. The at least one anisotropically shaped iron nitride crystal or grain can exhibit an aspect ratio of about 1.1 to about 50, e.g., about 1.4 to about 50, or 2.2 to about 50, or about 5 to about 50. As used herein, aspect ratio is defined as the ratio of the length of the longest dimension to the length of the shortest dimension of an anisotropic grain, where the shortest dimension is measured in a direction that is substantially orthogonal (e.g., orthogonal or nearly orthogonal (e.g., within about 5 degrees of orthogonal)) to the longest dimension. In some instances, the shortest dimension of the α″-Fe ₁₆ N ₂ crystallites or grains 84 is between about 5 nm and about 300 nm.

In some examples, the longest dimension of at least one anisotropically shaped iron nitride crystal or grain can be substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the direction of the applied magnetic field 32 and thus the direction of the uniaxial magnetic anisotropy. Similarly, the longest dimension of at least one anisotropically shaped iron nitride crystal or grain can be substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the easy axis of the magnetocrystalline anisotropy of the anisotropically shaped iron nitride crystal or grain. For example, for body-centered tetragonal (bct) Fe ₁₆ N ₂ and Fe, the (002) texture can be substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the longest dimension of the crystal or grain. In this manner, the shape anisotropy possessed by the anisotropically shaped iron nitride crystal or grain can contribute to the magnetic anisotropy of the material.

In some examples, the applied magnetic field can affect the properties of the grain boundaries, such as by hardening the grain boundaries. The applied magnetic field can promote the creation of pinning sites, such as dopant atoms or defects, located at or near the grain boundaries during the casting process (14), which can increase the hardness of the grain boundaries (e.g., regions within about 1 nm to about 100 nm of the grain boundaries). For example, the applied magnetic field can promote the migration of dopant atoms or defects within the grains to the grain boundaries.

In some examples, the external magnetic field 32 may be a static magnetic field generated by a DC mode electromagnet. The static magnetic field may not change as a function of time during the casting technique. The DC mode external magnetic field 32 may have a magnetic flux density of about 0.01 Tesla (T) to about 50 T. In some examples, the external magnetic field 32 may be at least 0.2 T. In some examples, the external magnetic field 32 may be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field 32 is about 5 T to about 10 T. In other examples, the external magnetic field 32 is about 8 T to about 10 T. In other examples, the external magnetic field 32 is a varying magnetic field generated by an AC mode electromagnet. The varying magnetic field may change as a function of time during the casting technique. The AC mode external magnetic field 32 can have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In some examples, the external magnetic field 32 can be at least about 0.2 T. In some examples, the external magnetic field 32 can be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field 32 is about 5 T to about 10 T. In other examples, the external magnetic field 32 is about 8 T to about 10 T.

In some examples, the external magnetic field 32 may be substantially uniform (e.g., uniform or nearly uniform (e.g., within about 5%)) throughout the RF furnace 22, or at least throughout the volume occupied by the crucible 26. In other examples, the external magnetic field 32 may vary as a function of position. For example, the external magnetic field 32 may vary along the direction of the external magnetic field (as shown by the direction of the arrow in FIG. 2). For example, the gradient can be from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter), such as from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 50 Tesla/meter (about 0.05 Tesla/millimeter), or from about 1 Tesla/meter (about 0.001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter). In some examples, the gradient can be monotonically increasing or decreasing in the magnetic flux density of the external magnetic field 32.

During casting of the iron and nitrogen-containing material in the presence of the external magnetic field 32 (14), the molten iron and nitrogen-containing material can be cooled and solidified. In some examples, this cooling process can be relatively slow, for example, the cooling can be triggered by turning off the heating of the RF furnace 22. In other examples, the molten iron and nitrogen-containing material can be quenched in a quench medium to more quickly cool and solidify the iron and nitrogen-containing material. The system of FIG. 2 optionally includes a quench medium 28. In some examples, the quench medium 28 can include water (room temperature, low temperature, or ice water), oil, brine, aqueous ammonia, or amide. The molten iron and nitrogen-containing material can be poured into the quench medium, or the quench medium can be circulated around the crucible 26 or the solidified (but still hot) iron and nitrogen-containing material. If used, the temperature of the quench medium 28 can be from about -269°C to about 210°C.

The cast material containing iron and nitrogen may include at least one iron nitride. In addition to iron and/or nitrogen, the at least one iron nitride may be, for example, FeN, Fe ₂ N (e.g., ξ-Fe ₂ N), Fe ₃ N (e.g., ε-Fe ₃ N), Fe ₄ N (e.g., γ'-Fe ₄ N and/or γ-Fe ₄ N), Fe ₂ N ₆ , α-Fe ₈ N, α″-Fe ₁₆ N ₂ , or FeN _x (where x is from about 0.05 to about 0.5). The cast material may then be subjected to further processing to convert at least some of the above types of iron nitride to α″-Fe ₁₆ N _2. Some examples of further processing are described below with respect to FIG. 11.

In some instances, rather than using the system shown in FIG. 2, other types of apparatus can be used to cast a mixture containing iron and nitrogen in the presence of an external magnetic field. FIG. 3 shows a schematic diagram of a system 40 including a crucible heating stage 42 that can be used to cast a material containing iron and nitrogen 46 in the presence of an external magnetic field. FIG. 4 shows a schematic diagram of one example of the crucible heating stage 42 shown in FIG. 3 in more detail.

As best shown in FIG. 3, the iron- and nitrogen-containing material 46 is encased by a coating material 48. The coating material 48 can be glass or another amorphous material having a melting point similar to glass. The coating material 48 can substantially encapsulate (e.g., encase or nearly encapsulate) the iron- and nitrogen-containing material 46. Because the coating material 48 is amorphous, it can tightly encase and stress the material. In this manner, the coating material 48 can facilitate the introduction of strain into the iron- and nitrogen-containing material 46, which can result in the formation of a material having a high saturation magnetization. The iron- and nitrogen-containing material 46 can be in the form of a wire, ribbon, film, or the like, prior to entering the crucible heating stage 42.

In the examples shown in Figures 3 and 4, the iron and nitrogen containing material 46 passes vertically through the crucible heating stage 42 from top to bottom in the figure. In other examples, the iron and nitrogen containing material 46 may pass vertically through the crucible heating stage 42 from bottom to top in the figure.

The crucible heating stage 42 defines an opening 56 through which the iron- and nitrogen-containing material 46 passes (e.g., through which a portion of the iron- and nitrogen-containing material 46 is disposed). In some examples, no portion of the crucible heating stage 42 contacts the iron- and nitrogen-containing material 46 during heating of the iron- and nitrogen-containing material 46. In some embodiments, this is advantageous because it reduces the risk of undesirable elements or species contacting and diffusing into the iron- and nitrogen-containing material 46. Undesirable elements or species may affect the properties of the iron- and nitrogen-containing material 46. Therefore, it may be desirable to reduce or limit contact between the iron- and nitrogen-containing material 46 and other materials.

The crucible heating stage 42 also includes an inductor 44 that surrounds at least a portion of the opening 56 defined by the crucible heating stage 42. The inductor 44 includes a conductive material through which an electric current can flow, such as aluminum, silver, or copper. The electric current passing through the inductor 44 can induce eddy currents in the iron- and nitrogen-containing material 46 by means of alternating current (AC), causing the iron- and nitrogen-containing material 46 to heat.

The iron and nitrogen containing material 46 is heated by eddy currents to form a molten iron and nitrogen containing material 46. In some examples, not shown in FIGS. 3 and 4, during the melting process, the iron and nitrogen containing molten material 46 is axially stretched such that the thickness or diameter of the iron and nitrogen containing molten material 46 is reduced relative to the iron and nitrogen containing solid material 46. During the melting process, the iron and nitrogen containing molten material 46 remains substantially encapsulated by the cladding material 48.

In some examples, the molten material containing iron and nitrogen may be drawn through an opening in the coil 50, which can define the cross-sectional size and shape of the cast material containing iron and nitrogen 52.

Optionally, to facilitate cooling of the iron- and nitrogen-containing cast material 52, the iron- and nitrogen-containing cast material 52 may be exposed to a cooling medium, such as water (room temperature, cold, or iced), oil, brine, aqueous ammonia, or an amide. In another example, the iron- and nitrogen-containing cast material 52 may be air-cooled.

During the melting and cooling portion of the casting process, the iron- and nitrogen-containing material 48 (solid, molten, and cast 52) is exposed to an external magnetic field 56 generated by a magnet 54. The external magnetic field 56 can affect grain nucleation and growth during cooling and solidification of the iron- and nitrogen-containing molten material 48 into the iron- and nitrogen-containing cast material 52, as described with respect to FIG. 2. Additionally or alternatively, the external magnetic field 56 can promote diffusion of nitrogen into interstitial spaces in the iron lattice, thereby reducing or substantially preventing (e.g., preventing or nearly preventing) nitrogen from diffusing out of the iron- and nitrogen-containing material. Because the external magnetic field 56 can affect nucleation density and defect density during the nucleation and growth process, the external magnetic field 56 can also affect the iron grain size and grain boundaries.

In some examples, the external magnetic field 56 may be similar or substantially the same (e.g., the same or nearly the same) as the external magnetic field 32 described with respect to FIG. 2. For example, the external magnetic field 56 may be a static magnetic field generated by a DC mode electromagnet and may have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In other examples, the external magnetic field 56 may be a fluctuating magnetic field generated by an AC mode electromagnet and may have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In some examples, the external magnetic field 56 can be at least 0.2 T. In some examples, the external magnetic field 56 may be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field 56 is about 5 T to about 10 T. In other examples, the external magnetic field 56 is about 8 T to about 10 T. In some examples, the external magnetic field 56 can be substantially uniform (e.g., uniform or nearly uniform (e.g., within about 5%)) across the entire crucible heating stage 42, or at least across the entire volume of the iron-nitrogen containing material 48. In other examples, the external magnetic field 56 can vary as a function of position. For example, the external magnetic field 56 can vary along the direction of the external magnetic field (as shown by the direction of the arrow in FIG. 3). For example, the gradient can be from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter), such as from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 50 Tesla/meter (about 0.05 Tesla/millimeter), or from about 1 Tesla/meter (about 0.001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter). In some examples, the gradient can be a monotonically increasing or decreasing flux density of the external magnetic field 56.

The iron and nitrogen containing cast material 52 may include at least one iron nitride, in addition to iron and/or nitrogen, such as FeN, Fe ₂ N (e.g., ξ-Fe ₂ N), Fe ₃ N (e.g., ε-Fe ₃ N), Fe ₄ N (e.g., γ'-Fe ₄ N and/or γ-Fe ₄ N), Fe ₂ N ₆ , α-Fe ₈ N, α″-Fe ₁₆ N ₂ , or FeN _x (where x is from about 0.05 to about 0.5). The cast material may then be subjected to further processing to convert at least some of the above types of iron nitrides to α″-Fe ₁₆ N _2. Some examples of further processing are described below with respect to FIG. 11.

FIG. 5 is a conceptual diagram illustrating another exemplary system 60 for belt casting an exemplary iron nitride workpiece in the presence of an external magnetic field. The belt casting system 60 may include an ingot chamber 62 containing a molten iron ingot 64 and is heated by a heating source, for example in the form of a heating coil 66. In some examples, the temperature of the molten iron ingot 64 in the ingot chamber 62 may be greater than about 1800 Kelvin (K; about 1526.85° C.). The pressure of the iron ingot 64 in the ingot chamber 62 may be about 0.06 MPa to about 0.12 MPa.

The ingot chamber 62 includes a nitrogen inlet 68 through which a nitrogen source is introduced into the molten iron ingot 64 to form the molten iron nitride mixture 70. Nitrogen may be provided through the nitrogen inlet 68 in a variety of forms or from a variety of sources. For example, nitrogen may be provided in the form of ammonia, ammonium azide, or urea, which is introduced through the nitrogen inlet 68 and then decomposes to release nitrogen atoms upon mixing with the molten iron in the molten iron nitride mixture 70.

In some examples, the nitrogen source may be provided to provide an approximately stoichiometric number of nitrogen atoms in the iron nitride mixture 70. The stoichiometric ratio of iron to nitrogen in Fe ₁₆ N ₂ is 8:1. Thus, the approximate number of iron atoms in the iron nitride mixture 70 can be determined, and a number of nitrogen atoms equal to about ⅛ (12.5%) of the iron atoms, for example, about 8 at.% to about 15 at.%, can be provided to the iron nitride mixture 70 through the nitrogen inlet 68.

The molten iron nitride mixture 70 flows out of the ingot chamber 62 through a nozzle head 72 to form an iron nitride strip 74. The iron nitride strip 74 is fed into a gap region between the surfaces of counter-rotating first and second pinch rollers 76a and 76b (collectively, "pinch rollers 76"). In some examples, the distance from the nozzle head 72 to the surface of the pinch rollers 76 may be from about 1 mm to about 50 mm, for example about 4 mm.

In some examples, the rotational speed of the first pinch roller 76a and the second pinch roller 76b can range from about 10 revolutions per minute (rpm) to 5000 rpm, and the rotational speed of the rollers 76 can be approximately the same. In some examples, the pinch roller 76 is actively cooled, for example using water cooling, to maintain the surface of the roller 76 at a temperature lower than that of the iron nitride strip 74 and aid in cooling and casting the iron nitride strip 74. For example, the temperature of the pinch roller 76 may be maintained at about 300 K (about 26.85° C.) to about 400 K (about 126.85° C.). The pressure exerted by the pinch roller 76 on the iron nitride strip 74 can be about 0.04 MPa to about 0.1 MPa.

After the iron nitride strip 74 is pressed between the pinch rollers 76 and cooled, the iron nitride strip 74 forms textured iron nitride sheets 78a and 78b. In some examples, the textured iron nitride sheets 78a and 78b (collectively "textured iron nitride sheets 78") may form textured iron nitride ribbons (either individually or after compression of multiple textured iron nitride sheets 78) having at least one dimension (e.g., thickness) of about 1 μm to about 10 mm, for example, about 5 μm to about 1 cm. Each of the textured iron nitride sheets 78 may include, for example, a (002) or (004) crystal texture. In other words, a major surface of each of the textured iron nitride sheets 78 may be parallel to the (002) or (004) surfaces of all or substantially all of the iron crystals in each of the textured iron nitride sheets 78. By using textured iron nitride sheets 78a and 78b in which all or substantially all (e.g., all or nearly all (e.g., greater than ₉₅ %)) of the iron crystallites have their crystal axes substantially aligned (e.g., aligned or nearly aligned (e.g., within about 5 degrees of being aligned)) in a subsequent process, the anisotropy formed when forming the Fe8N and _{Fe16N two} -phase domains can be substantially aligned between the crystallites.

During the belt casting technique, the magnet 80 can generate an external magnetic field 82 to which at least the molten iron nitride mixture 70 and the iron nitride strip 74 are exposed. The external magnetic field 82 can affect grain nucleation and growth during cooling and solidification of the molten iron nitride mixture 70 into the iron nitride strip 74, as described above with respect to FIG. 2. Additionally or alternatively, the external magnetic field 82 can promote diffusion of nitrogen into the interstitial spaces of the iron lattice, thereby reducing or substantially preventing (e.g., preventing or nearly preventing) nitrogen from diffusing out of the iron and nitrogen-containing material. Because the external magnetic field 82 can affect nucleation density and defect density during the nucleation and growth process, the external magnetic field 56 can also affect the iron grain size and grain boundaries.

In some examples, the external magnetic field 82 may be similar or substantially the same as the external magnetic field 32 described with respect to FIG. 2. For example, the external magnetic field 82 may be a static magnetic field generated by a DC mode electromagnet and may have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In some examples, the external magnetic field 82 may be at least about 0.2 T. In some examples, the external magnetic field 82 may be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field 82 may be about 5 T to about 10 T. In other examples, the external magnetic field 82 is about 8 T to about 10 T. In other examples, the external magnetic field 82 may be a fluctuating magnetic field generated by an AC mode electromagnet and may have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In some examples, the external magnetic field 82 can be at least about 0.2 T. In some examples, the external magnetic field 82 can be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field 82 is between about 5 T and about 10 T. In other examples, the external magnetic field 82 is between about 8 T and about 10 T. In some examples, the external magnetic field 82 can be substantially uniform (e.g., uniform or nearly uniform (within about 5%)) throughout the belt casting system 60, or at least throughout the volume of the molten iron nitride mixture 70 and the iron nitride strip 74. In other examples, the external magnetic field 82 can vary as a function of position. For example, the external magnetic field 82 can vary along the direction of the external magnetic field (indicated by the direction of the arrow in FIG. 5). For example, the gradient can be from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter), such as from about 0.01 Tesla/meter to about 50 Tesla/meter (about 0.05 Tesla/millimeter), or from about 1 Tesla/meter (about 0.001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter). In some examples, the gradient can be a monotonically increasing or decreasing flux density of the external magnetic field 82.

The iron nitride strip 74 may include at least one iron nitride, in addition to iron and/or nitrogen, such as FeN, Fe ₂ N (e.g., ξ-Fe ₂ N), Fe ₃ N (e.g., ε-Fe ₃ N), Fe ₄ N (e.g., γ'-Fe ₄ N and/or γ-Fe ₄ N), Fe ₂ N ₆ , α-Fe ₈ N, α″-Fe ₁₆ N ₂ , or FeN _x (where x is from about 0.05 to about 0.5). The iron nitride strip 74 may then be subjected to further processing to convert at least some of the above types of iron nitrides to α″-Fe ₁₆ N _2. Some examples of further processing are described below with respect to FIG. 11.

In the above examples, a casting technique was described in which a material containing a mixture of iron and nitrogen was exposed to a magnetic field during the casting technique. The present disclosure also describes a compression technique for joining multiple workpieces containing α″-Fe ₁₆ N ₂ phase domains in the presence of an external magnetic field. FIG. 6 is a conceptual diagram illustrating an α″-Fe ₁₆ N ₂ unit cell. As shown in FIG. 6, in the α″-Fe ₁₆ N ₂ phase, the N atoms are aligned along the (002) (iron) crystallographic plane. The iron nitride unit cell is distorted such that the length of the unit cell along the <001> axis is about 6.28 angstroms (Å), while the length of the unit cell along the <010> and <100> axes is about 5.72 Å. When the α″-Fe ₁₆ N ₂ unit cell is in a distorted state, it can be referred to as a body-centered tetragonal (bct) unit cell. When the α″-Fe ₁₆ N ₂ unit cell is in a distorted state, the <001> axis can be called the c-axis of the unit cell. This c-axis can be the easy axis of magnetization of the α″-Fe ₁₆ N ₂ unit cell. In other words, the α″-Fe ₁₆ N ₂ crystal exhibits magnetic anisotropy.

α″-Fe ₁₆ N ₂ has high saturation magnetization and magnetic anisotropy constants. The high saturation magnetization and magnetic anisotropy constants result in a magnetic energy product that can be higher than rare earth magnets. For example, experimental evidence from thin film α″-Fe ₁₆ N ₂ permanent magnets suggests that bulk Fe ₁₆ N ₂ permanent magnets have desirable magnetic properties including a high energy product as high as about 134 Megagauss*Oersted (MGOe), which is about twice the energy product of NdFeB (which has an energy product of about 60 MGOe). Calculations and experiments indicate that the magnetocrystalline anisotropy of α″-Fe ₁₆ N ₂ can be about 1.0-2.0×10 ⁷ erg/cm ^3. α″-Fe ₁₆ N ₂ also has a relatively high theoretical magnetic saturation moment of about 2.9 Bohr magnetons per iron atom μ _B /Fe. Furthermore, iron and nitrogen are abundant elements and therefore relatively cheap and easy to procure.

Without wishing to be bound by theory, three types of anisotropy may contribute to the magnetic anisotropy energy or magnetic anisotropy field of α″-Fe ₁₆ N ₂ or other iron-based magnetic materials. These three types of anisotropy include magnetocrystalline anisotropy, shape anisotropy, and distortion anisotropy. As discussed above, magnetocrystalline anisotropy may be related to the distortion of the bcc iron crystal lattice to the bct iron nitride crystal lattice shown in FIG. 6 . Shape anisotropy may be related to the shape of the iron nitride crystals or grains, or the shape of the iron nitride workpiece. For example, as shown in FIG. 7 , an α″-Fe ₁₆ N ₂ crystal or grain 84 may exhibit a longest dimension (substantially parallel to the z-axis of FIG. 7 , although orthogonal x-y-z axes are shown for ease of illustration only). The α″-Fe ₁₆ N ₂ crystals or grains 84 may exhibit a shortest dimension (e.g., substantially parallel to the x-axis or y-axis of FIG. 7 ). The shortest dimension may be measured in a direction perpendicular to the longest axis of the α″-Fe ₁₆ N ₂ crystals or grains 84.

In some examples, the α″-Fe ₁₆ N ₂ crystals or grains 84 can exhibit an aspect ratio of about 1.1 to about 50, e.g., about 1.4 to about 50, or 2.2 to about 50, or about 5 to about 50. In some examples, the shortest dimension of the α″-Fe ₁₆ N ₂ crystals or grains 84 is about 5 nm to about 300 nm.

Strain anisotropy may be associated with strain on the α″-Fe ₁₆ N ₂ or other iron-based magnetic material. In some examples, the α″-Fe ₁₆ N ₂ grains are disposed or embedded in a matrix that includes iron or other type of iron nitride (e.g., Fe ₄ N) grains. The α″-Fe ₁₆ N ₂ grains may have a different thermal expansion coefficient than the iron or other type of iron nitride grains. This difference may introduce strain in the α″-Fe ₁₆ N ₂ grains due to differential dimensional changes in the α″-Fe ₁₆ N ₂ grains and the iron or other type of iron nitride grains during thermal processing. Alternatively or additionally, a material or workpiece may be subjected to strain, either mechanically or by exposure to an applied magnetic field, during processing to form the α″-Fe ₁₆ N ₂ grains, and at least some of such strain may remain in the processed material or workpiece. Annealing can result in a redistribution of the internal stresses and local microstructure of the sample to reduce the magnetoelastic energy under stress. The domain structure under strain anisotropy depends on the magnetoelastic energy, magnetostatic energy and exchange energy.

FIG. 8 is a conceptual diagram illustrating an exemplary workpiece 86 including a plurality of α″-Fe ₁₆ N ₂ crystals or grains 84 in a matrix 88 of another material. As shown in FIG. 8 , each of the α″-Fe ₁₆ N ₂ crystals or grains 84 exhibits an anisotropic shape. Further, the magnetic easy axis of each α″-Fe ₁₆ N ₂ crystal or grain of the α″-Fe ₁₆ N ₂ crystals or grains 84 is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the respective longest dimension of each α″-Fe ₁₆ N ₂ crystal or grain. In some examples, the magnetic easy axis of each α″-Fe ₁₆ N ₂ crystal or grain is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to each of the other respective longest dimensions (and thus substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to each of the other respective longest dimensions). In some examples, this can be accomplished by casting the material used to form workpiece 86 in the presence of an applied magnetic field, as described with respect to Figures 1-5. In this manner, workpiece 86 can have structural properties that result in magnetocrystalline anisotropy, shape anisotropy, and strain anisotropy, all of which contribute to the anisotropy field of workpiece 86.

Figure 9 shows an exemplary hysteresis curve for workpiece 86. The hysteresis curve shown in Figure 9 indicates that workpiece 86 has magnetic anisotropy because the coercive force (x-axis intercept) of workpiece 86 when a magnetic field is applied parallel to the c-axis direction in Figure 8 is different from the coercive force (x-axis intercept) of workpiece 86 when a magnetic field is applied parallel to the a-axis and b-axis directions in Figure 8.

It can be difficult to directly produce a bulk material containing an α″-Fe ₁₆ N _two -phase domain. An alternative technique described herein involves forming smaller materials containing an α″-Fe ₁₆ N _two -phase domain and then joining (or consolidating) the smaller materials to form a bulk magnetic material containing an α″-Fe ₁₆ N _two -phase domain. FIG. 10 is a flow diagram illustrating an exemplary technique for consolidating multiple workpieces containing at least one α″-Fe ₁₆ N _two -phase domain to form a bulk magnetic material.

In some examples, instead of consolidating multiple work-pieces including at least one α″-Fe ₁₆ N ₂ phase domain to form a bulk magnetic material, the technique of FIG. 10 may include consolidating multiple work-pieces including at least one iron-based phase domain including uniaxial magnetic anisotropy, such as distorted iron, Fe ₁₆ C ₂ or iron and at least one of B, O, P, Y, Mn, Co, Cr, Si, Al, etc.

The technique of FIG. 10 includes mixing (92) a plurality of workpieces including iron nitride with a binder material. At least some of the plurality of workpieces including iron nitride may include at least one α″-Fe ₁₆ N _two -phase domain. In some examples, each of the plurality of workpieces including iron nitride may include at least one α″-Fe ₁₆ N _two- phase domain. Additionally, the plurality of workpieces may include other iron nitride phase domains (e.g., domains of FeN, Fe ₂ N (e.g., ξ-Fe ₂ N), Fe ₃ N (e.g., ε-Fe ₃ N), Fe ₄ N (e.g., γ′-Fe ₄ N and/or γ-Fe ₄ N), Fe ₂ N ₆ , α-Fe ₈ N, or FeN _x (where x is from about 0.05 to about 0.5)), iron phase domains, etc.

The plurality of workpieces can include any shape and size. In some examples, the workpieces include one dimension that is longer than the other dimensions of the respective workpiece. Examples of workpieces having a dimension that is longer than the other dimensions include fibers, wires, filaments, cables, films, thick films, foils, ribbons, sheets, and the like. In other examples, the workpieces may not have a dimension that is longer than the other dimensions of the workpiece. For example, the workpieces can include particles or powders, such as spheres, cylinders, platelets, flakes, regular polyhedrons, irregular polyhedrons, and any combination thereof. Examples of suitable regular polyhedrons include tetrahedrons, hexahedrons, octahedrons, decahedrons, dodecahedrons, and the like, non-limiting examples of which include cubes, prisms, pyramids, and the like.

Binder materials include any material that can be pressed together with multiple workpieces to form a cohesive bulk material. In some examples, binders include resins, waxes, or low melting point metals. Low melting point metals include, for example, zinc (Zn), tin (Sn), bismuth (Bi), gallium (Ga), sodium (Na), or lithium (Li). Exemplary resins include natural or synthetic resins, including ion exchange resins, such as those available under the trade name Amberlite™ from The Dow Chemical Company, Midland, Michigan; epoxy resins, such as bismaleimide-triazine (BT)-epoxy; polyacrylonitrile; polyesters; silicones; prepolymers; polyvinyl butyral; urea-formaldehyde; and the like.

The mixture including the plurality of workpieces and the binder may then be exposed to an external magnetic field (94). The external magnetic field may have a predetermined orientation with respect to the mixture including the plurality of workpieces and the binder. This predetermined orientation may be used to define the magnetization direction of the bulk material. For example, when the mixture including the plurality of workpieces and the binder is initially mixed, the respective easy axes of each of the plurality of workpieces (e.g., the c-axes of α″-Fe ₁₆ N ₂ ) may be substantially randomly oriented (randomly oriented or near randomly oriented). When a bulk material is formed in which the respective easy axes of each of the plurality of workpieces are substantially randomly oriented, the magnetic anisotropy of the bulk material may be relatively small, which may degrade the magnetic properties (e.g., energy product) of the bulk magnetic material.

The magnetic anisotropy of the bulk magnetic material can be increased by substantially aligning at least some of the easy axes of each of the workpieces using an external magnetic field, thereby improving the magnetic properties (e.g., energy product) of the bulk magnetic material. The external magnetic field can also enable the magnetization direction of the bulk material to be defined, for example, by substantially aligning at least some of the easy axes of each of the plurality of workpieces. For example, the mixture of the binder and the plurality of workpieces can be placed in a mold that defines the near net shape of the final magnetic material, and an external magnetic field can be oriented in a selected direction relative to the mold to define the magnetization direction of the bulk material.

In some examples, the external magnetic field may be a static magnetic field generated by a DC mode electromagnet and may have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In some examples, the external magnetic field can be at least 0.2 T. In some examples, the external magnetic field may be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field is about 5 T to about 10 T. In other examples, the external magnetic field is about 8 T to about 10 T.

In other examples, the external magnetic field may be a varying magnetic field generated by an AC mode electromagnet and may have a magnetic flux density of about 0.01 Tesla to about 50 Tesla. In some examples, the external magnetic field 82 can be at least about 0.2 T. The external magnetic field 82 may be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the external magnetic field 82 is about 5 T to about 10 T. In other examples, the external magnetic field 82 is about 8 T to about 10 T. In some examples, the external magnetic field 82 is substantially uniform throughout the belt casting system 60, or at least throughout the volume of the molten iron nitride mixture 70 and the iron nitride strip 74. In other examples, the external magnetic field 82 may vary as a function of position. For example, the external magnetic field 82 may vary along the direction of the external magnetic field (indicated by the direction of the arrow in FIG. 5). For example, the gradient can be from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter), such as from about 0.01 Tesla/meter (about 0.00001 Tesla/millimeter) to about 50 Tesla/meter (about 0.05 Tesla/millimeter), or from about 1 Tesla/meter (about 0.001 Tesla/millimeter) to about 1000 Tesla/meter (about 1 Tesla/millimeter). In some examples, the gradient can be a monotonically increasing or decreasing flux density of the external magnetic field 82.

While the mixture including the binder and the plurality of workpieces is exposed to the external magnetic field (94), the mixture may be compressed (96) to bond the binder and the plurality of workpieces and form a bulk magnetic material. Compressing the mixture including the binder and the plurality of workpieces (96) may include applying a pressure to the mixture. For example, the pressure may be from about 1 megapascal (MPa) to about 100 gigapascals (GPa) at room temperature. Compressing the mixture including the binder and the plurality of workpieces may be performed at a relatively low temperature (e.g., from about -268.93°C (the boiling point of liquid helium at atmospheric pressure) to about room temperature (about 23°C)). Alternatively, compressing the mixture including the binder and the plurality of workpieces may be performed at a relatively high temperature (e.g., from about room temperature (about 23°C) to about 210°C). The product of the compressing step may be a bulk magnetic material including α″-Fe ₁₆ N _two- phase domains.

In some examples, the casting and compacting processes described herein can be performed together in the same overall technique for forming a bulk magnetic material comprising an α″-Fe ₁₆ N _two- phase domain. FIG. 11 is a flow diagram illustrating an exemplary method for forming a bulk magnetic material comprising an α″-Fe ₁₆ N _two -phase domain from a feedstock comprising iron and nitrogen. The technique of FIG. 11 includes forming a molten mixture comprising iron and nitrogen (102). This step is substantially similar to step (12) described with respect to FIG. 1. The technique of FIG. 11 also includes casting the molten mixture comprising iron and nitrogen in the presence of an external magnetic field (104). This step may be similar or substantially similar to step (14) described with respect to FIG. 1.

The technique of FIG. 11 also optionally includes pressing (106) the iron and nitrogen containing material. The iron and nitrogen containing material may be pressed to obtain a predetermined size of the iron and nitrogen containing material. During the pressing step, the temperature of the iron and nitrogen containing material is maintained below about 250° C., and the iron and nitrogen containing material may be subjected to a pressure of about 5 tons to 50 tons depending on the desired final dimension (e.g., thickness or diameter) of the iron and nitrogen containing material. In some examples, when the pressing process is completed, the iron and nitrogen containing material may be in the shape of a workpiece having dimensions of about 0.001 mm to about 50 mm (e.g., diameter of about 0.1 mm to about 50 mm for wire, thickness of about 0.001 mm to about 5 mm for ribbon) in one or more axes. The iron and nitrogen containing material may include at least one Fe ₈ N iron nitride phase domain after pressing is completed.

In some examples, the technique also includes quenching (108) the iron and nitrogen containing material, if desired. Quenching can fix the crystal structure and phase composition of the iron and nitrogen containing material. For example, quenching can promote the formation of Fe ₈ N phase domains in the iron and nitrogen containing material. In some examples, during the quenching process, the iron and nitrogen containing material can be heated to a temperature greater than 650° C. for about 0.5 hours to about 20 hours. In some examples, the temperature of the iron and nitrogen containing material can be rapidly reduced below the martensite temperature (Ms) of the workpiece alloy. For example, for Fe ₁₆ N ₂ , the martensite temperature (Ms) is about 250° C. Media used for quenching include liquids, such as water, brine (salt concentration of about 1% to about 30%), non-aqueous liquids or solutions, such as oil, or liquid nitrogen. In other examples, the quench medium can include a gas, such as nitrogen gas, at a flow rate of about 1 sccm to about 1000 sccm. In other examples, the quench medium can include solids such as salt, sand, etc. In some examples, a workpiece containing iron and nitrogen may be cooled at a rate of over 50° C. per second during the quenching process. In some examples, the quenching process can be assisted by a magnetic field and/or an electric field.

The technique of FIG. 11 may further include stretching (or drawing) and annealing (110), (112) the iron- and nitrogen-containing material. This stretching and annealing process may convert at least some of the Fe ₈ N iron nitride phase domains in the iron- and nitrogen-containing material to Fe ₁₆ N ₂ phase domains. Various strain-inducing devices may be used to apply strain to the iron- and nitrogen-containing material. For example, the iron- and nitrogen-containing material may be received (e.g., wrapped) by a first set of rollers and a second set of rollers, and the rollers of the sets may be rotated in opposite directions to apply a tension force to the iron- and nitrogen-containing material. In another example, both ends of the iron- and nitrogen-containing material may be held by mechanical grippers, e.g., clamps, and the mechanical grippers may be moved away from each other to apply a tension force to the iron- and nitrogen-containing material.

In some examples, the iron-nitrogen containing material may be stretched along a direction that is substantially parallel (e.g., parallel or nearly parallel (e.g., within about 5 degrees of parallel)) to the <001> axis of at least one iron crystal in the iron-nitrogen containing material. The strain inducing device may stretch the iron-nitrogen containing material to a particular elongation. For example, the strain for the iron-nitrogen containing material may be about 0.3% to about 12%. In other examples, the strain for the iron-nitrogen containing material may be less than about 0.3% or greater than about 12%. In some examples, applying a certain strain to the iron-nitrogen containing material results in a substantially similar strain in the individual unit cells of the iron (or iron nitride), resulting in a unit cell that is about 0.3% to about 12%.

While the iron- and nitrogen-containing material is strained, the iron- and nitrogen-containing material may be heated to anneal the iron- and nitrogen-containing material (112). The iron- and nitrogen-containing material may be annealed by heating the iron- and nitrogen-containing material to a temperature between about 100° C. and about 250° C., such as between about 120° C. and about 200° C. Annealing the iron- and nitrogen-containing material while straining it may facilitate converting at least some of the iron nitride phase domains to α″-Fe ₁₆ N _two- phase domains.

The annealing process may continue for a predetermined time sufficient to diffuse the nitrogen atoms into the appropriate interstitial spaces. In some examples, the annealing process lasts from about 20 hours to about 100 hours, such as from about 40 hours to about 60 hours. In some examples, the annealing process may be performed under an inert atmosphere, such as, for example, Ar, to reduce or substantially prevent oxidation of the iron. In some embodiments, the temperature is held substantially constant while annealing the iron and nitrogen comprising material. The stretching (110) and annealing (112) of the iron and nitrogen comprising material may result in a magnetic material comprising at least one α″-Fe ₁₆ N _two -phase domain.

In some examples, the iron and nitrogen-containing material may be exposed to an external magnetic field during drawing (110) and annealing (112) of the iron and nitrogen-containing material. Annealing the iron nitride material in the presence of an applied magnetic field may promote the formation of the Fe ₁₆ N ₂ phase in the iron nitride material. Increasing the volume fraction of the Fe ₁₆ N ₂ phase may improve the magnetic properties of the iron nitride-containing magnetic material. The improved magnetic properties may include, for example, coercivity, magnetization, and magnetic orientation. In some examples, the applied magnetic field may be at least 0.2 Tesla (T). The temperature at which the magnetic field annealing is performed may depend, at least in part, on the additional element additions to the iron nitride-based composition and the approach used to initially synthesize the iron nitride-based composition. In some examples, the magnetic field may be at least about 0.2 T, at least about 2 T, at least about 2.5 T, at least about 6 T, at least about 7 T, at least about 8 T, at least about 9 T, at least about 10 T, or higher. In some examples, the magnetic field is about 5 T to about 10 T. In other examples, the magnetic field is between about 8 T and about 10 T. Further details regarding annealing materials containing iron and nitrogen are provided in U.S. Provisional Application No. 62/019,046, filed June 30, 2014.

The technique of FIG. 11 may include compressing (114) a plurality of workpieces of magnetic material including at least one α″-Fe ₁₆ N _two -phase domain with a binder material to form a bulk magnetic material. This step may be similar or substantially the same as the technique described with reference to FIG. 10.

The technique of FIG. 11 may further include shaping (116) the bulk magnetic material (if desired). This shaping process may include, for example, slicing or grinding the surface of the bulk magnetic material to form the bulk magnetic material into a desired final shape. Finally, the technique of FIG. 11 may further include magnetizing (118) the bulk magnetic material (if desired). Thus, the technique of FIG. 11 describes an exemplary technique for forming a bulk magnetic material including α″-Fe ₁₆ N _two -phase domains.

Clause 1: A method comprising casting an iron-containing material in the presence of an applied magnetic field to form a workpiece comprising at least one iron-based phase domain having uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T).

Clause 2: The method of clause 1, wherein casting the iron-containing material includes casting the iron- and nitrogen-containing material in the presence of an applied magnetic field to form a workpiece including at least one iron nitride phase domain.

Clause 3: The method of clause 1, wherein casting the material comprising iron comprises casting a material comprising iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn or Al in the presence of an applied magnetic field to form a workpiece comprising at least one phase domain having uniaxial magnetic anisotropy.

Clause 4: The method of any one of clauses 1 to 3, wherein casting the iron-containing material in the presence of an applied magnetic field includes casting the iron-containing material in the presence of an applied magnetic field to form a workpiece including at least one anisotropically shaped iron-based crystal grain, the at least one anisotropically shaped iron-based crystal grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension of the anisotropic crystal grain to the length of the shortest dimension, the longest dimension and the shortest dimension being substantially orthogonal.

Item 5: The method of item 4, wherein at least one anisotropically shaped iron-based crystal grain exhibits a shortest dimension of about 5 nm to about 300 nm.

Item 6: The method of items 4 or 5, wherein the at least one anisotropically shaped iron-based crystal grain comprises a plurality of anisotropically shaped iron-based crystal grains, and the major axes of each of the plurality of anisotropically shaped iron-based crystal grains are oriented substantially parallel to each other.

Item 7: The method according to any one of items 4 to 6, in which the major axis of each of the plurality of anisotropically shaped iron-based crystal grains is oriented substantially parallel to the direction of the applied magnetic field.

Item 8: The method according to any one of items 4 to 7, in which, for each anisotropically shaped iron-based crystal grain, the respective easy axes of magnetocrystalline anisotropy are substantially parallel to the respective longest axes.

Item 9: The method according to any one of items 1 to 8, wherein the strength of the applied magnetic field exceeds about 0.02 T.

Item 10: The method according to any one of items 1 to 8, wherein the strength of the applied magnetic field exceeds about 2.5 T.

Item 11: The method according to any one of items 1 to 8, wherein the strength of the applied magnetic field exceeds about 9 T.

Item 12: The method according to any one of items 1 to 11, wherein the strength of the applied magnetic field is less than about 50 T.

Item 13: The method according to any one of items 1 to 12, wherein the material further comprises at least one dopant.

Clause 14: The method of clause 13, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.

Item 15: The method of item 13, wherein the at least one dopant comprises at least one of B, C, P, Si, or O.

Item 16: The method of item 2, wherein the concentration of nitrogen in the material is about 8 atomic percent (at.%) to about 9 at.% prior to casting.

Clause 17: Casting
heating the iron-containing mixture to form a molten iron-containing mixture; and
cooling the iron-containing molten mixture to form a workpiece;
17. The method of any one of claims 1 to 16, comprising:

Item 18: The method of item 17, wherein cooling the molten mixture includes quenching the molten mixture in a quench medium.

Item 19: The method of item 18, wherein the quench medium comprises at least one of water, ice water, brine, oil, aqueous ammonia, or an amide.

Item 20: The method of any one of items 17 to 19, wherein heating the mixture containing iron comprises heating the mixture containing iron in the presence of an applied magnetic field, and cooling the molten mixture containing iron comprises cooling the molten mixture containing iron in the presence of an applied magnetic field.

Item 21: The method according to any one of items 17 to 20, wherein heating the mixture containing iron includes heating the mixture containing iron in a crucible using a high-frequency furnace.

Clause 22: The method of any one of clauses 17 to 20, wherein heating the iron-containing mixture includes heating the iron-containing mixture in a cold crucible, and the mixture is substantially encapsulated in the coating material.

Item 23: The method of any one of items 17 to 20, wherein cooling the iron-containing molten mixture includes cooling the iron-containing molten mixture between cooling rollers to form a workpiece.

Clause 24: A method comprising compressing a plurality of workpieces, each workpiece including at least one iron-based phase domain including uniaxial magnetic anisotropy, in the presence of an applied magnetic field to form a bulk material including a plurality of iron-based phase domains including uniaxial magnetic anisotropy, the applied magnetic field having a magnitude of at least 0.01 Tesla (T), the applied magnetic field defining a magnetization direction of the bulk material.

Clause 25: The method of clause 24, wherein the at least one iron-based phase domain comprising uniaxial magnetic anisotropy comprises at least one α″-Fe ₁₆ N _2- phase domain.

Item 26: The method according to item 24 or 25, wherein at least one iron-based phase domain having uniaxial magnetic anisotropy comprises at least one of a body-centered tetragonal iron phase domain or a phase domain having a body-centered tetragonal crystal structure and comprising iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Al, or Zn.

Clause 27: The method of clauses 24-26, wherein each workpiece comprises at least one iron-based phase domain having uniaxial magnetic anisotropy and at least one anisotropically shaped iron-based grain, the at least one anisotropically shaped iron-based grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension to the length of the shortest dimension, the longest dimension and the shortest dimension being substantially perpendicular.

28. The method of claim 27, wherein at least one anisotropically shaped iron-based crystal grain exhibits a shortest dimension of about 5 nm to about 300 nm.

Clause 29: The method of clause 27 or clause 28, wherein the at least one anisotropically shaped iron-based crystal grain comprises a plurality of anisotropically shaped iron-based crystal grains, and the major axes of each of the plurality of anisotropically shaped iron-based crystal grains are oriented substantially parallel to each other.

Item 30: The method according to any one of items 27 to 29, wherein the major axis of each of the plurality of anisotropically shaped iron-based crystal grains is oriented substantially parallel to the direction of the applied magnetic field.

Item 31: The method of any one of items 27 to 30, wherein for each anisotropically shaped iron-based crystal grain, the respective easy axes of magnetocrystalline anisotropy are substantially parallel to the respective longest axes.

Item 32: The method according to any one of items 24 to 31, wherein the strength of the applied magnetic field is greater than about 0.02 T.

Item 33: The method according to any one of items 24 to 31, wherein the strength of the applied magnetic field is greater than about 2.5 T.

Item 34: The method according to any one of items 24 to 31, wherein the strength of the applied magnetic field is greater than about 9 T.

Item 35: The method according to any one of items 24 to 34, wherein the strength of the applied magnetic field is less than about 50 T.

Item 36: The method of any one of items 24 to 35, wherein at least one of the plurality of workpieces further comprises at least one dopant.

Clause 37: The method of clause 36, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.

Item 38: The method of item 36, wherein the at least one dopant comprises at least one of B, C, P, Si, or O.

Clause 39: The method of any one of clauses 24 to 38, wherein the applied magnetic field promotes substantial alignment of the easy magnetic axes of at least some of the plurality of workpieces.

Clause 40: The method of any one of clauses 24 to 39, wherein compressing the plurality of workpieces includes mixing the plurality of workpieces with at least one of a resin, a wax, or a low melting point metal to form a mixture, and pressing the mixture to form a bulk material.

Item 41: The method of item 40, wherein pressing the mixture includes pressing the mixture at a pressure of about 1 MPa to about 100 GPa.

Clause 42: The method of clause 40 or clause 41, wherein pressing the mixture comprises cold pressing the mixture at a temperature of about 4.2 Kelvin to about 295 Kelvin.

Clause 43: The method of clause 40 or clause 41, wherein pressing the mixture comprises hot pressing the mixture at a temperature of about 295 Kelvin to about 533 Kelvin.

Clause 44: The method of any one of clauses 40 to 43, wherein mixing the plurality of workpieces with at least one of a resin, a wax, or a low melting point metal includes mixing the plurality of workpieces with a low melting point metal material, the low melting point metal including at least one of Zn, Sn, Bi, Ga, Na, or Li.

Item 45: The method of any one of items 24 to 44, wherein one of the workpieces is at least one of a powder, a ribbon, or a wire.

Clause 46: The method of any one of clauses 24 to 43, further comprising the method of any one of clauses 1 to 22, wherein the workpiece is one of a plurality of workpieces.

Clause 47: An apparatus configured to carry out the method according to any one of clauses 1 to 46.

Item 48: A workpiece formed by the method according to any one of items 1 to 23.

Item 49: A bulk material formed by the method of any one of items 24 to 46.

Item 50: The bulk material according to item 49, wherein the bulk material is a bulk permanent magnet.

51. A method comprising: casting a material comprising at least one of nickel, iron and cobalt in the presence of an applied magnetic field to form a workpiece comprising at least one nickel-, iron- or cobalt-based phase domain having uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T).

Item 52: The method of item 51, wherein the metal includes iron.

53. The method of claim 52, wherein casting the iron-containing material comprises casting the iron- and nitrogen-containing material in the presence of an applied magnetic field to form a workpiece comprising at least one iron nitride phase domain.

Item 54: The method according to any one of items 51 to 53, wherein the material containing at least one of nickel, iron, or cobalt further contains at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al.

Item 55: The method according to any one of items 51 to 54, wherein the material further comprises at least one dopant, and the at least one dopant comprises at least one of B, C, P, Si, or O.

Item 56: The method of any one of items 51 to 55, further comprising compressing the plurality of workpieces.

57. A workpiece comprising at least one anisotropically shaped iron-based grain, the at least one anisotropically shaped iron-based grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension of the anisotropic grain to the length of the shortest dimension, the longest dimension and the shortest dimension being substantially perpendicular.

Clause 58: The workpiece of clause 57, wherein at least one anisotropically shaped iron-based grain exhibits a shortest dimension of about 5 nm to about 300 nm.

59. The workpiece of claim 57 or 58, wherein the at least one anisotropically shaped iron-based crystal grain comprises a plurality of anisotropically shaped iron-based crystal grains, the major axes of each of the plurality of anisotropically shaped iron-based crystal grains being oriented substantially parallel to one another.

Item 60: The workpiece of any one of items 57 to 59, further comprising at least one iron-based phase domain having uniaxial magnetic anisotropy, and wherein the longest dimension of at least one anisotropically shaped iron-based crystal grain is substantially parallel to the direction of the uniaxial magnetic anisotropy.

Item 61: A workpiece according to any one of items 57 to 60, in which at least one anisotropically shaped iron-based crystal grain comprises iron nitride.

Item 62: The workpiece of item 61, wherein the iron nitride comprises α″-Fe ₁₆ N ₂ .

Clause 63: A workpiece according to any one of clauses 57 to 60, in which at least one anisotropically shaped iron-based crystal grain contains iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al.

Item 64: The workpiece according to any one of items 57 to 63, further comprising at least one dopant.

Item 65: The workpiece of item 64, wherein at least one anisotropically shaped iron-based grain contains the dopant.

Clause 66: The workpiece of clause 64 or 65, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.

Item 67: The workpiece according to item 64 or 65, wherein the at least one dopant comprises at least one of B, C, P, Si, or O.

Item 68: A workpiece according to any one of items 57 to 67, in which for each anisotropically shaped iron-based crystal grain, the respective easy axes of magnetocrystalline anisotropy are substantially parallel to the respective longest axes.

Item 69: A bulk permanent magnet comprising at least one anisotropically shaped iron-based crystal grain, the at least one anisotropically shaped iron-based crystal grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension of the anisotropic crystal grain to the length of the shortest dimension, the longest dimension and the shortest dimension being substantially perpendicular.

Item 70: A bulk permanent magnet as described in Item 69, in which at least one anisotropically shaped iron-based crystal grain exhibits a shortest dimension of about 5 nm to about 300 nm.

Item 71: A bulk permanent magnet as described in item 69 or 70, in which at least one anisotropically shaped iron-based crystal grain includes a plurality of anisotropically shaped iron-based crystal grains, the major axes of each of the plurality of anisotropically shaped iron-based crystal grains being oriented substantially parallel to each other.

Item 72: A bulk permanent magnet according to any one of items 69 to 71, further comprising at least one iron-based phase domain having uniaxial magnetic anisotropy, and wherein the longest dimension of at least one anisotropically shaped iron-based crystal grain is substantially parallel to the direction of the uniaxial magnetic anisotropy.

Item 73: A bulk permanent magnet according to any one of items 69 to 72, wherein the at least one anisotropically shaped iron-based crystal grain comprises iron nitride.

Item 74: The bulk permanent magnet of item 73, wherein the iron nitride comprises α″-Fe ₁₆ N ₂ .

Item 75: A bulk permanent magnet according to any one of items 69 to 72, in which the at least one anisotropically shaped iron-based crystal grain contains iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al.

Item 76: A bulk permanent magnet according to any one of items 69 to 75, further comprising at least one dopant.

Item 77: A bulk permanent magnet as described in Item 76, in which at least one anisotropically shaped iron-based crystal grain contains the dopant.

Item 78: A bulk permanent magnet according to item 76 or 77, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.

Item 79: A bulk permanent magnet according to item 76 or 77, wherein the at least one dopant comprises at least one of B, C, P, Si or O.

Item 80: A bulk permanent magnet according to any one of items 69 to 79, in which, for each anisotropically shaped iron-based crystal grain, the respective easy axes of magnetocrystalline anisotropy are substantially parallel to the respective longest axes.

Figure 12 shows exemplary X-ray diffraction spectra from iron nitride material cast with and without an external magnetic field. The darker traces show the phase composition when cast in the presence of a magnetic field. The lighter traces show the phase composition when cast without an applied magnetic field. The nitrogen concentrations in the samples averaged about 5 at.% and about 8 at.%. The samples were heated at about 650°C for about 4 hours with and without a 9 T magnetic field. The samples were cast in ice water. The cooling rate was estimated to be about 200°C/sec. Table 2 shows the changes in the peaks shown in Figure 12 after magnetic casting.

Various examples have been described. These and other examples are within the scope of the following claims.
Some of the embodiments of the invention related to the present invention are shown below.
[Aspect 1]
1. A workpiece comprising at least one anisotropically shaped iron-based grain, the at least one anisotropically shaped iron-based grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of a length of a longest dimension of the anisotropic grain to a length of a shortest dimension thereof, the longest and shortest dimensions being substantially orthogonal.
[Aspect 2]
2. The workpiece of claim 1, wherein the at least one anisotropically shaped iron-based grain exhibits a shortest dimension of about 5 nm to about 300 nm.
[Aspect 3]
3. The workpiece of claim 1 or 2, wherein the at least one anisotropically shaped iron-based grain comprises a plurality of anisotropically shaped iron-based grains, the major axes of each of the plurality of anisotropically shaped iron-based grains being oriented substantially parallel to one another.
[Aspect 4]
4. The workpiece of any one of aspects 1-3, further comprising at least one iron-based phase domain comprising uniaxial magnetic anisotropy, wherein the longest dimension of the at least one anisotropically shaped iron-based grain is substantially parallel to a direction of the uniaxial magnetic anisotropy.
[Aspect 5]
A workpiece as recited in any one of aspects 1-4, wherein the at least one anisotropically shaped iron-based grain comprises iron nitride.
[Aspect 6]
6. The workpiece of claim 5, wherein the iron nitride comprises α″-Fe ₁₆ N ₂ .
[Aspect 7]
A workpiece as recited in any one of aspects 1-4, wherein the at least one anisotropically shaped iron-based grain comprises iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al.
[Aspect 8]
The workpiece of any one of the preceding aspects, further comprising at least one dopant.
[Aspect 9]
9. The workpiece of claim 8, wherein the at least one anisotropically shaped iron-based grain comprises the dopant.
[Aspect 10]
10. The workpiece of claim 8 or 9, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.
[Aspect 11]
10. The workpiece of claim 8 or 9, wherein the at least one dopant comprises at least one of B, C, P, Si, or O.
[Aspect 12]
A workpiece as claimed in any one of aspects 1-11, wherein for each anisotropically shaped iron-based grain, the respective easy axis of magnetocrystalline anisotropy is substantially parallel to the respective longest axis.
[Aspect 13]
A bulk permanent magnet comprising a plurality of workpieces, at least one of the plurality of workpieces comprising the workpiece of any one of aspects 1-12.
[Aspect 14]
14. The bulk permanent magnet of aspect 13, wherein each of the plurality of workpieces comprises iron nitride.
[Aspect 15]
15. An article comprising the bulk permanent magnet of aspect 13 or 14.
[Aspect 16]
16. The article of claim 15, wherein the article comprises an electric motor, a generator, a sensor, an actuator, a component of an automobile, or a component of a wind turbine.
[Aspect 17]
1. A method comprising: casting an iron-containing material in the presence of an applied magnetic field to form a workpiece comprising at least one iron-based phase domain comprising uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T).
[Aspect 18]
20. The method of claim 17, wherein casting the material comprising iron comprises casting a material comprising iron and nitrogen in the presence of an applied magnetic field to form a workpiece comprising at least one iron nitride phase domain.
[Aspect 19]
18. The method of claim 17, wherein casting the material comprising iron comprises casting a material comprising iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al in the presence of an applied magnetic field to form a workpiece comprising at least one phase domain having uniaxial magnetic anisotropy.
[Aspect 20]
20. The method of any one of aspects 17-19, wherein casting the iron-containing material in the presence of the applied magnetic field comprises casting the iron-containing material in the presence of the applied magnetic field to form a workpiece comprising at least one anisotropically shaped iron-based grain, wherein the at least one anisotropically shaped iron-based grain has an aspect ratio of about 1.1 to about 50, wherein the aspect ratio is defined as the ratio of a length of a longest dimension to a length of a shortest dimension of an anisotropic grain, and wherein the longest dimension and shortest dimension are substantially orthogonal.
[Aspect 21]
21. The method of aspect 20, wherein the at least one anisotropically shaped iron-based grain exhibits a shortest dimension of about 5 nm to about 300 nm.
[Aspect 22]
22. The method of claim 20 or 21, wherein the at least one anisotropically shaped iron-based grain comprises a plurality of anisotropically shaped iron-based grains, the major axes of each of the plurality of anisotropically shaped iron-based grains being oriented substantially parallel to one another.
[Aspect 23]
Aspects 23. The method of any one of aspects 20 to 22, wherein a major axis of each of the plurality of anisotropically shaped iron-based crystalline grains is oriented substantially parallel to the direction of the applied magnetic field, and for each anisotropically shaped iron-based crystalline grain, a respective easy axis of magnetocrystalline anisotropy is substantially parallel to its respective longest axis.
[Aspect 24]
Aspect 24. The method of any one of aspects 17-23, wherein the strength of the applied magnetic field is greater than about 0.02 T.
[Aspect 25]
Aspect 24. The method of any one of aspects 17-23, wherein the strength of the applied magnetic field is greater than about 2.5 T.
[Aspect 26]
Aspect 24. The method of any one of aspects 17-23, wherein the strength of the applied magnetic field is greater than about 9 T.
[Aspect 27]
27. The method of any one of aspects 17-26, wherein the applied magnetic field has a gradient of about 0.01 Tesla/meter to about 1000 Tesla/meter.
[Aspect 28]
28. The method of any one of aspects 17 to 27, wherein the material further comprises at least one dopant.
[Aspect 29]
29. The method of claim 28, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.
[Aspect 30]
29. The method of claim 28, wherein the at least one dopant comprises at least one of B, C, P, Si, or O.
[Aspect 31]
19. The method of embodiment 18, wherein prior to casting, the concentration of nitrogen in the material is from about 8 atomic percent (at.%) to about 9 at.%.
[Aspect 32]
The casting,
heating the iron-containing mixture to form a molten iron-containing mixture; and
cooling the iron-containing molten mixture to form a workpiece;
32. The method of any one of aspects 17 to 31, comprising:
[Aspect 33]
33. The method of claim 32, wherein cooling the molten mixture comprises quenching the molten mixture in at least one of water, ice water, brine, oil, aqueous ammonia, or an amide.
[Aspect 34]
33. The method of claim 32, wherein cooling the molten mixture comprising iron comprises cooling the molten mixture comprising iron between cooling rollers to form a workpiece.
[Aspect 35]
35. The method of any one of aspects 32-34, wherein heating the mixture comprising iron comprises heating the mixture comprising iron in the presence of an applied magnetic field, and cooling the molten mixture comprising iron comprises cooling the molten mixture comprising iron in the presence of an applied magnetic field.
[Aspect 36]
35. The method of any one of aspects 32-34, wherein heating the mixture comprising iron comprises heating the mixture comprising iron in a crucible using a radio frequency furnace.
[Aspect 37]
35. The method of any one of aspects 32-34, wherein heating the iron-containing mixture comprises heating the iron-containing mixture in a cold crucible, wherein the mixture is substantially encapsulated within a coating material.
[Aspect 38]
Aspects 38. The method of any one of aspects 17-37, further comprising compressing the plurality of workpieces to form a bulk material.
[Aspect 39]
39. The method of claim 38, wherein compressing the plurality of workpieces to form the bulk material comprises compressing the plurality of workpieces in the presence of an applied magnetic field to form a bulk material comprising a plurality of iron-based phase domains comprising uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least about 0.01 Tesla (T).
[Aspect 40]
1. A method comprising: compressing a plurality of workpieces, each workpiece comprising at least one iron-based phase domain comprising uniaxial magnetic anisotropy, in the presence of an applied magnetic field to form a bulk material comprising a plurality of iron-based phase domains comprising uniaxial magnetic anisotropy, the applied magnetic field having a strength of at least 0.01 Tesla (T), the applied magnetic field defining a magnetization direction of the bulk material.
[Aspect 41]
41. The method of aspect 40, wherein the at least one iron-based phase domain comprising uniaxial magnetic anisotropy comprises at least one α″-Fe ₁₆ N ₂ -phase domain.
[Aspect 42]
42. The method of claim 40 or 41, wherein the at least one iron-based phase domain comprising uniaxial magnetic anisotropy comprises at least one of a body-centered tetragonal iron phase domain, or a phase domain having a body-centered tetragonal crystal structure and comprising iron and at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al.
[Aspect 43]
43. The method of any one of aspects 40 to 42, wherein the at least one iron-based phase domain comprising uniaxial magnetic anisotropy comprises at least one anisotropically shaped iron-based grain, the at least one anisotropically shaped iron-based grain having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of the length of the longest dimension to the length of the shortest dimension, the longest dimension and the shortest dimension being substantially orthogonal.
[Aspect 44]
44. The method of claim 43, wherein the at least one anisotropically shaped iron-based grain exhibits a shortest dimension of about 5 nm to about 300 nm.
[Aspect 45]
45. The method of claim 43 or 44, wherein the at least one anisotropically shaped iron-based grain comprises a plurality of anisotropically shaped iron-based grains, the major axes of each of the plurality of anisotropically shaped iron-based grains being oriented substantially parallel to one another.
[Aspect 46]
Aspects 46. The method of any one of aspects 43 to 45, wherein a major axis of each of the plurality of anisotropically shaped iron-based crystalline grains is oriented substantially parallel to the direction of an applied magnetic field, and for each anisotropically shaped iron-based crystalline grain, a respective easy axis of magnetocrystalline anisotropy is substantially parallel to its respective longest axis.
[Aspect 47]
Aspect 47. The method of any one of aspects 40-46, wherein the strength of the applied magnetic field is greater than about 0.02 T.
[Aspect 48]
Aspect 47. The method of any one of aspects 40-46, wherein the strength of the applied magnetic field is greater than about 2.5 T.
[Aspect 49]
Aspect 47. The method of any one of aspects 40-46, wherein the strength of the applied magnetic field is greater than about 9 T.
[Aspect 50]
50. The method of any one of aspects 40-49, wherein the applied magnetic field has a gradient of about 0.01 Tesla/meter to about 1000 Tesla/meter.
[Aspect 51]
Aspects 40-51, wherein at least one of the plurality of workpieces further comprises at least one dopant.
[Aspect 52]
52. The method of claim 51, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal.
[Aspect 53]
52. The method of embodiment 51, wherein the at least one dopant comprises at least one of B, C, P, Si, or O.
[Aspect 54]
Aspects 44. The method of any one of aspects 40-53, wherein the applied magnetic field promotes substantially aligning easy axes of at least some of the plurality of workpieces.
[Aspect 55]
Aspects 40-55. The method of any one of aspects 40-54, wherein compressing the plurality of workpieces comprises mixing the plurality of workpieces with at least one of a resin, a wax, or a low melting point metal to form a mixture, and pressing the mixture to form a bulk material.
[Aspect 56]
56. The method of claim 55, wherein pressing the mixture comprises pressing the mixture at a pressure of about 1 MPa to about 100 GPa.
[Aspect 57]
57. The method of claim 55 or 56, wherein pressing the mixture comprises cold pressing the mixture at a temperature of from about 4.2 Kelvin to about 295 Kelvin.
[Aspect 58]
57. The method of claim 55 or 56, wherein pressing the mixture comprises hot pressing the mixture at a temperature of about 295 Kelvin to about 533 Kelvin.
[Aspect 59]
Aspects 59. The method of any one of aspects 55-58, wherein mixing the plurality of workpieces with at least one of a resin, a wax, or a low melting point metal comprises mixing the plurality of workpieces with a low melting point material, the low melting point metal comprising at least one of Zn, Sn, Bi, Ga, Na, or Li.
[Aspect 60]
A method according to any one of aspects 40 to 59, wherein a workpiece of the plurality of workpieces comprises at least one of a powder, a ribbon, or a wire.
[Aspect 61]
Aspects 40-60, further comprising the method of any one of aspects 17-39, wherein the workpiece formed by the method of any one of aspects 17-39 is one of the plurality of workpieces.
[Aspect 62]
An apparatus configured to carry out the method of any one of aspects 17 to 61.
[Aspect 63]
A workpiece formed by the method of any one of aspects 17-39.
[Aspect 64]
A bulk material formed by the method of any one of aspects 40-61.
[Aspect 65]
65. The bulk material of aspect 64, wherein the bulk material is a bulk permanent magnet.
[Aspect 66]
66. The bulk permanent magnet of aspect 65, wherein each of the plurality of workpieces comprises iron nitride.
[Aspect 67]
67. An article comprising the bulk permanent magnet of claim 65 or 66.
[Aspect 68]
68. The article of claim 67, wherein the article constitutes an electric motor, a generator, a sensor, an actuator, an automotive component, or a wind turbine component.
[Aspect 69]
1. A method comprising: casting a material comprising at least one of nickel, iron and cobalt in the presence of an applied magnetic field to form a workpiece comprising at least one nickel-, iron- or cobalt-based phase domain comprising uniaxial magnetic anisotropy, wherein the applied magnetic field has a strength of at least about 0.01 Tesla (T).
[Aspect 70]
70. The method of embodiment 69, wherein the metal comprises iron.
[Aspect 71]
71. The method of embodiment 70, wherein casting the iron-containing material comprises casting the iron- and nitrogen-containing material in the presence of an applied magnetic field to form a workpiece comprising at least one iron nitride phase domain.
[Aspect 72]
72. The method of any one of aspects 69-71, wherein casting a material comprising at least one of nickel, iron, or cobalt forms a workpiece comprising at least one of C, B, O, P, Y, Mn, Co, Cr, Si, Zn, or Al in the presence of an applied magnetic field, the workpiece comprising at least one phase domain having uniaxial magnetic anisotropy.
[Aspect 73]
Aspect 73. The method of any one of aspects 69-72, wherein the material further comprises at least one dopant, the at least one dopant comprising at least one of B, C, P, Si, or O.
[Aspect 74]
74. The method of any one of aspects 69-73, further comprising compressing the plurality of work pieces.
[Aspect 75]
75. The method of any one of aspects 69-74, wherein the applied magnetic field has a gradient of about 0.01 Tesla/meter to about 1000 Tesla/meter.
[Aspect 76]
A workpiece formed by the method of any one of aspects 69-75.
[Aspect 77]
77. A bulk material comprising a plurality of workpieces according to aspect 76.
[Aspect 78]
78. The bulk material of aspect 77, wherein the bulk material is a bulk permanent magnet.
[Aspect 79]
79. An article comprising the bulk permanent magnet of claim 78.
[Aspect 80]
80. The article of claim 79, wherein the article constitutes an electric motor, a generator, a sensor, an actuator, an automotive component, or a wind turbine component.

Claims (15)

A bulk magnetic material comprising a plurality of workpieces and a binder,
the workpiece comprises a plurality of anisotropically shaped iron-based grains each comprising iron nitride and at least one iron-based phase domain comprising uniaxial magnetic anisotropy, at least one of the anisotropically shaped iron-based grains having an aspect ratio of about 1.1 to about 50, the aspect ratio being defined as the ratio of a length of a longest dimension of the at least one anisotropically shaped iron-based grain to a length of a shortest dimension thereof, the longest dimension and the shortest dimension being substantially orthogonal;
the longest dimension of the at least one anisotropically shaped iron-based grain is parallel to a direction of uniaxial magnetic anisotropy;
the workpiece further comprises at least one dopant;
The workpiece is a bulk magnetic material cast from a mixture comprising iron and nitrogen in the presence of an applied magnetic field . The bulk magnetic material of claim 1, wherein the at least one dopant comprises at least one of Al, Mn, La, Cr, Co, Ti, Ni, Zn, Zr, Ca, or a rare earth metal. The bulk magnetic material of claim 1, wherein the at least one dopant comprises at least one of Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Ca, Pt, Au, Sm, C, Pb, W, Ga, Y, Mg, Hf, or Ta. 2. The bulk magnetic material of claim 1, wherein the iron nitride comprises α″-Fe ₁₆ N ₂ . The bulk magnetic material of claim 1, wherein the at least one anisotropically shaped iron-based grain comprises the dopant and further comprises at least one dopant selected from the group consisting of B, C, P, Si, and O. The bulk magnetic material of claim 1, wherein for each anisotropically shaped iron-based crystal grain, the respective easy axes of magnetocrystalline anisotropy are substantially parallel to the respective longest axes. The bulk magnetic material of claim 1, wherein the binder comprises a resin, a wax, or a low melting point metal. The bulk magnetic material of claim 7, wherein the low melting point metal is selected from zinc (Zn), tin (Sn), bismuth (Bi), gallium (Ga), sodium (Na), or lithium (Li). The bulk magnetic material of claim 7, wherein the resin is selected from ion exchange resin, epoxy resin, polyacrylonitrile, polyester, silicone, prepolymer, polyvinyl butyral, or urea-formaldehyde. 10. A method of forming the bulk magnetic material of claim 1 comprising:
mixing a plurality of workpieces with a binder to form a mixture;
applying an external magnetic field to the mixture; and compressing the mixture to form the bulk magnetic material. The method of claim 10, wherein applying the external magnetic field to the mixture substantially aligns the easy magnetic axes of the workpieces. The method of claim 10, wherein the external magnetic field is a static magnetic field having a magnetic flux density of about 0.01 Tesla to about 50 Tesla. The method of claim 10, wherein the external magnetic field is a varying magnetic field having a magnetic flux density of about 0.01 Tesla to about 50 Tesla. The method of claim 10, wherein the pressure in the step of compressing the mixture is from about 1 megapascal (MPa) to about 100 gigapascal (GPa). The method of claim 14, wherein the temperature of the step of compressing the mixture is about 23°C to about 210°C.